1. Field of the Invention
The present invention relates to wireless communication systems, wireless base stations and mobile stations, preferably, to a mobile wireless communication system employing the W-CDMA (UMTS) communication scheme and to the wireless base stations and mobile stations used in such a system.
2. Description of the Related Art
Currently, HSDPA (High Speed Downlink Packet Access) is being implemented as one data transport scheme using wireless communication (see Non-Patent Document 1). HSDPA is a scheme that allows high-speed downlink packet transport, with a maximum transfer rate of approximately 14 Mbps being considered possible.
HSDPA is characterized in that it employs an adaptive modulation and coding (AMC) scheme, switching for example between the QPSK modulation scheme and 16-QAM scheme adaptively according to the wireless environment between the base station and mobile station.
Furthermore, HSDPA employs an H-ARQ (Hybrid Automatic Repeat ReQuest) scheme. Under H-ARQ, when a mobile station detects an error in data received from a base station, the mobile station makes a retransmission request to the base station in question. The base station performs retransmission of data upon receiving this retransmission request, and the mobile station performs error correction decoding using both the already received data and the retransmitted received data. In this way, H-ARQ reduces the number of retransmissions by effectively utilizing already received data, even if it contains errors.
The main wireless channels used in HSDPA include HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel) and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
HS-SCCH and HS-PDSCH are both downlink (i.e. in the direction from the base station to the mobile station) shared channels. HS-SCCH is a control channel for transmitting various parameters relating to the data transmitted on HS-PDSCH. The various parameters include, for example, modulation scheme information indicating which modulation scheme is to be used, the allocated spreading code number (code number), information on the rate matching pattern applied before transmission, etc.
Furthermore, HS-DPCCH is an uplink (in the direction from the mobile station to the base station) dedicated control channel, which is used by the mobile station for instance for sending the result of reception for data received from the base station via HS-PDSCH in the form of an ACK signal or NACK signal (response signal), depending respectively on whether the reception was or was not successful. If the mobile station fails to receive data (if the received data has a CRC error, etc.), a NACK signal will be transmitted from the mobile station as a retransmission request and the base station will accordingly perform retransmission control. Furthermore, if the wireless base station cannot receive either an ACK or a NACK signal (in the case of DTX), the fact that the mobile station has sent neither an ACK nor a NACK signal can also be a retransmission request for purposes of retransmission control.
In addition, HS-DPCCH is used by a mobile station to transmit, to a base station, reception quality information, determined by the mobile station for signals received from a base station, in the form of CQI (Channel Quality Indicator) information (e.g., SIR). The base station modifies the downlink transmission format based on the received CQI information. In other words, if the CQI information indicates that the downlink wireless environment is good, the base station would switch the transmission format to a modulation scheme that allows higher speed data transmission, while if the CQI information indicates that the downlink wireless environment is not good, the base station would switch the transmission format to a modulation scheme which transmits data at a lower rate (i.e., the base station performs adaptive modulation).
Channel Structure
Next, the channel configuration of HSDPA will be described.
FIG. 1 is a drawing that illustrates the channel configuration of HSDPA. Since W-CDMA employs a code division multiplexing scheme, the individual channels are separated by code.
First, the channels that have not been explained will be briefly described.
CPICH (Common Pilot Channel) is a downlink common channel that is transmitted to all mobile stations in a wireless zone (cell).
CPICH is a channel for transmitting a so-called pilot signal, and is used by the mobile station for channel estimation, cell search, and as timing reference for other downlink physical channels in the same cell.
Next, the timing relationship of the channels will be described using FIG. 1.
As shown in the drawing, in each channel, one frame (10 ms) consists of 3×5=15 slots (each slot comprises a 2560 chip length). As described above, CPICH is used as a reference for other channels, so the head of the P-CCPCH and HS-SCCH frames is aligned with the head of the CPICH frame. Here, the head of the HS-PDSCH frame is delayed by 2 slots relative to HS-SCCH, etc., which is to make it possible for the mobile station to perform demodulation of HS-PDSCH with the modulation scheme corresponding to the received modulation type after receiving modulation type information via HS-SCCH. Furthermore, HS-SCCH and HS-PDSCH comprise sub-frames of 3 slots.
HS-DPCCH is an uplink channel, which contains a slot (1 slot long) used by the mobile station for transmitting an ACK/NACK signal, which is a response for acknowledgement of reception, to the base station approximately 7.5 slots after the HS-PDSCH reception.
Furthermore, HS-PDCCH is used for periodically transmitting CQI information as feedback for adaptive modulation control to the base station. Here, the transmitted CQI information is for example calculated based on the reception environment (e.g. the SIR determination result for CPICH) as determined in the period from 4 slots until 1 slot before the CQI transmission.
Matters relating to HSDPA as discussed above are disclosed for instance in 3G TS 25.212 (3rd Generation Partnership Project: Technical Specification; Group Radio Access Network; Multiplexing and channel coding (FDD)) V6.2.0 (June 2004).